1. Field of the Invention
The present invention relates to a magnetron, and more particularly, to a capacitor of a magnetron, designed to have excellent withstand voltage and capacitance, thereby enhancing noise shielding efficiency, and allowing reduction in filling amount of insulating filler in the capacitor together with size reduction of the capacitor.
2. Discussion of the Related Art
Generally, a magnetron is applied to microwave ovens, plasma illuminating devices, driers, and other high frequency systems. In the magnetron, thermal electrons are emitted to a cathode by application of power, and generate microwaves by electromagnetic field. Then, the microwaves are output as a heat source to heat a target.
A conventional magnetron will be described with reference to FIGS. 1 to 4.
Referring to FIG. 1, the overall construction of the magnetron will be described.
The magnetron generally comprises a high frequency generator for generating microwaves by an applied voltage, an output portion for emitting the microwaves generated from the high frequency generator, and an input portion for applying a the voltage to the high frequency generator.
The high frequency generator of the magnetron comprises upper and lower plate-shaped yokes 11a and 11b, an anode cylinder 12, cooling fins 13, upper and lower magnetic poles 14a and 14b, an A-shaped seal member 15a, an F-shaped seal member 15b, a ceramic stem 16, magnets 17a and 17b, vanes 21, and a cathode 22.
The anode cylinder 12 is located in an inner space defined between the upper and lower yokes 11a and 11b. 
Each of the cooling fins 13 is connected at one end to the anode cylinder 12 and at the other end to the upper or lower yoke plate 11a or 11b. The cooling fins 13 act to dissipate heat from the anode cylinder 12 to the upper and lower yokes 11a and 11b. 
The upper and lower magnetic poles 14a and 14b are disposed to upper and lower ends of the anode cylinder 12, respectively. The A-shaped seal member 15a is equipped to surround an outer surface of the upper magnetic pole 14a, and the F-shaped seal member 15b is equipped to surround an outer surface of the lower magnetic pole 14a. The magnets 17a and 17b are equipped to the outer surfaces of the upper and lower magnetic poles.
The upper and lower magnetic poles 14a and 14b, the A-shaped seal member 15a and the F-shaped seal member 15b, and the magnets 17a and 17b are symmetrically equipped to the upper and lower ends of the anode cylinder 12, respectively.
The lower end of the F-shaped seal member 15b is opened, and the ceramic stem 16 is equipped thereto. The ceramic stem 16 is penetrated with an outer connecting lead 25, which is connected to a center lead 23 and a side lead 24.
The anode cylinder 12, the A-shaped seal member 15a, the F-shaped seal member 15b, and the ceramic stem 16 close a space from which the microwaves are generated.
The anode cylinder 12 has the vane 21 equipped therein, and is formed at the center of the vane 21 with a chamber 21a where the microwaves are generated. The chamber 21a of the vane is equipped with the cathode 22 to which the center lead 23 is inserted. At this time, the vane 21 acts as a positive electrode, and the cathode 22 acts as a negative electrode. The microwaves are generated by interaction of the vane and the cathode.
The output portion of the magnetron comprises an antenna feeder 31, an A-shaped ceramic member 32, and an antenna cap 33.
The antenna feeder 31 is connected to the vane 21, and the A-shaped ceramic member 32 is located between an upper end of the A-shaped seal member 15a and the antenna cap 33. Thus, the microwaves generated from the chamber 21a of the vane 21 and the cathode 22 are guided by the antenna feeder 31, and are then emitted to the outside through the A-shaped ceramic member 32.
The input portion of the magnetron comprises a filter box 40, a capacitor 50, and a choke coil 60.
The filter box 40 is fixed to a lower end of the high frequency generator. The capacitor 50 is fixed to the filter box 40 while being connected to the choke coil 60, which is connected to the outer connecting lead 25 while being located inside the filter box 40.
The filter box 40 is spaced a predetermined distance for insulation from the choke coil 60, a coupled portion between the outer connecting lead 25 and the choke coil 60, and the outer connecting lead 25. Moreover, the filter box 40 is made of an electrically conductive material, such as a steel plate, so as to prevent the microwaves from being leaked to the outside.
The capacitor 50 will be described with reference to FIG. 2.
The capacitor 50 comprises an insulating case 51 fixedly inserted into the filter box 40, an insulating base 52 equipped to one end of the insulating case 51, two central conductors 53 inserted into the insulating base 52, a dielectric material 54 surrounding the central conductors 53 within the insulating case 51, insulating filler 55 filled in the insulating case 51, and a ground plate 56 equipped to the one end of the insulating case 51 while being grounded to the filter box 40.
After the central conductors 53 and the dielectric material 54 are fixed in the insulting case 51, the insulating case is filled with the insulating filler 55, and the insulating filler 55 is cured for a predetermined period of time (about 10 hours). The insulating filler 55 includes an epoxy resin.
The dielectric members constituting the capacitor will be described with reference to FIGS. 3 and 4.
The dielectric members 54 are disposed between the outer surfaces of the central conductors 52 and the insulating case 51 so as to face each other. The dielectric members 54 consist of barium titanate, BaTiO3.
Each of the dielectric members 54 is substantially formed in a semicircular shape, and is formed with inner and outer electrodes 54a and 54b on inner and outer surfaces thereof, respectively. Here, the inner and outer electrodes 54a and 54b are formed in semicircular shapes.
The inner and outer electrodes 54a and 54b are formed by plating a material having excellent electric conductivity, such as silver, on the surfaces of the electrodes. Here, the inner electrode 54a contacts the rod-shaped central conductor 52, and the outer electrode 54b is connected to the ground plate 56. The dielectric members 54 have predetermined withstand voltage and capacitance.
In order to produce a capacitor having a higher capacitance with a reduced size, it is advantageous to increase the withstand voltage and capacitance of the dielectric members 54. Here, the withstand voltage and capacitance of the dielectric members 54 are proportional to the dielectric constant ε of the dielectric members, effective surface areas of the inner and outer electrodes 54a and 54b, and wire diameters of the central conductors 53, but inversely proportional to the distance between the inner electrode and the outer electrode. Here, the dielectric constant ε is determined by a dielectric material, the effective surface areas are defined by heights and widths of the respective electrodes, and the wire diameter of the central conductors is defined by the radius a of the inner electrode.
The capacitances of the dielectric members 54 are varied according to the shapes thereof. Moreover, when the dielectric members have a higher withstand voltage, a capacitor can be manufactured to have a large capacitance with a reduced size by reducing the distance between the inner electrode 54a and the outer electrode 54b. 
Meanwhile, the ground plate 56 extends to the outside of the insulating case 51, and is grounded to the filter box 40. As a result, the inner and outer electrodes 54a and 54b, and the dielectric members 54 are grounded while repeating charge and discharge of electrons through the ground plate 56.
Operation of the magnetron constructed as described above will now be described as follows.
When power is applied to the magnetron, a predetermined voltage is supplied to the central conductors 53 of the capacitor 50. At this time, the dielectric members 54 have predetermined withstand voltage and capacitance.
The dielectric members 54 perform charge and discharge of electrons through the ground plate 56, and stabilize overvoltage surges applied to the capacitor. The capacitor supplies the stabilized voltage to the leads 23 and 24 through the outer connecting lead 25. Additionally, direct current is generated by interaction between the capacitor 50 and the choke coil 60, thereby shielding noise.
Electrons are emitted from the cathode 22 to the vane 21, so that microwaves are generated from the chamber of the vane. Then, the microwaves are guided to the outer portion by the antenna feeder 31 connected to the vane 21, and radiated through the A-shaped ceramic member.
However, the capacitor for the conventional magnetron has problems as follows.
Firstly, although the dielectric members are formed to have the semicircular shapes in order to increase the effective surface areas of the dielectric members, the outer electrode is formed to have an undesirably enlarged surface area compared to that of the inner electrode. That is, the outer electrode has the undesirably enlarged surface area compared to an effective surface area thereof. Thus, the size, in particular, a width W, of the capacitor is increased, and the amount of epoxy resin required to fill the insulating case is undesirably increased, thereby increasing the time for curing the epoxy resin. As a result, there are problems of increasing a time for manufacturing the products, a price of the products, and the size of the capacitor.
Secondly, the wire diameter of the central conductors is also increased in order to increase the withstand voltage and capacitance of the capacitor. However, in order to increase the wire diameter of the central conductors, the diameter of the central conductors must be greatly increased. In this case, costs for manufacturing the central conductors are increased, so that the sizes of the central conductors and the capacitor are increased together with an increase of a filling amount of the epoxy resin.
Thirdly, since the dielectric members have the semicircular shapes, the outer diameter of the dielectric members is remarkably increased when increasing a distance b-c between the inner electrode and the outer electrode. As a result, as the size of the dielectric members is remarkably increased, the size of the capacitor and the filling amount of the epoxy resin are increased.